Alpha Centauri is our closest neighboring star system, and its three members are the three nearest stars to the Sun. Alpha Centauri A and B lie about 4.37 light-years away; Proxima Centauri, also called Alpha Centauri C, is slightly nearer, at about 4.25 light-years. A light-year measures distance: even their light takes more than four years to reach us.
To the unaided eye, A and B merge into the bright southern star commonly called Alpha Centauri. A telescope can separate the pair under suitable conditions. They orbit each other in roughly 80 years. With Beta Centauri they form the Southern Pointers, a useful landmark near the Southern Cross. Proxima is a much fainter red dwarf and requires a telescope.
Proxima appears about 2.2 degrees from the bright pair, roughly four full-Moon widths across the sky. It really is widely separated: about 13,000 times the Earth–Sun distance, or 0.2 light-years, from A/B. Gravity can bind stars across that enormous gap because their relative motion is slow enough for Proxima to remain in orbit. An orbital solution estimates one circuit at roughly 550,000 years. The system is so close to us that its wide spacing is spread noticeably across our sky.
Our neighbors are also moving past us through space. Proxima is currently approaching the Sun along our line of sight at about 22 kilometres per second, or 49,000 miles per hour. That is a radial velocity, not its complete three-dimensional speed. Most of the approach is shared motion of the stellar system; it is distinct from Proxima’s slow orbit around A/B.
Proxima has not reached its closest approach to the Sun. A published trajectory estimate places that passage roughly 27,000 years in the future, at about 3.1 light-years, followed by increasing distance. The figures are approximate predictions. The nearest-star ranking changes over time as stellar trajectories and orbital positions change.
Sources: Kervella, Thévenin and Lovis, “Proxima’s orbit around Alpha Centauri” (2017), and the Observatoire de Paris account of that work; García-Sánchez and colleagues, “Stellar encounters with the solar system” (2001), for the closest-approach estimate; ESO’s annotated Alpha Centauri and Proxima sky image.
